Arginine vasotocin (AVT) plays a critical role in the expression of social behaviors in a wide variety of non-mammalian vertebrates. AVT is also known to stimulate reproductive behavior in the presence of testotserone (T) in males. However, the way in which AVT and T interact to produce male sexual behavior is unclear. This study will test the overarching hypothesis that T facilitates AVT induced male courtship behavior by modulating the AVT system in the brain of the green treefrog, Hyla cinerea. First, the distribution of AVT immunoreactivity (-ir) will be assessed and in males exposed to a social stimulus. Next, AVT-ir will be examined in males treated with an androgen receptor blocker or an estrogen receptor blocker. I will next determine the effects of the successful receptor antagonist on AVT-ir in males exposed to a social stimulus. Finally, I will examine AVT mRNA using in situ hybddization in control males compared to gonadectomized males, as changes in immunoreactivity can reflect either changes in release or synthesis. The second experiment will entail examining whether a seasonal difference exists in the AVT system of males in parallel with behavioral differences, and testing whether hormones delivered at the end of the breeding season will prevent the seasonal decline in AVT-ir. Lastly, if time permits, I will do double-label immunohistochemistry with AVT and either the androgen receptor or estrogen receptor, in an effort to determine if co-localization occurs between the two neurochemicals. This experiment will provide valuable information on where in the brain these two hormones are likely to act to influence courtship behavior in the green treefrog.